THE NGC-281 WEST CLUSTER .1. STAR-FORMATION IN PHOTOEVAPORATING CLUMPS

The NGC 281 West molecular cloud is an excellent test case for studyin g star formation in the clumpy interface between a Hn region and a gia nt molecular cloud. We present here a study based on new high resoluti on radio and near-infrared data. Using the IRAM 30-meter telescope, we have mapped the interface in the (CO)-O-18 (2-->1), (CO)-O-18 (1-->0) , and (CS)-S-34 (3-->2) transitions with FWHP beamwidths less than or equal to 22 ''. We have imaged the same region with the VLA in the 20, 6, and 2 cm continuum bands to obtain complementary maps of the ioniz ed gas distribution with angular resolutions less than or equal to 13 ''. In addition, we have obtained near-infrared J and K'-band images t o detect young stars in the interface. The 30-meter data shows the mol ecular gas is concentrated into three clumps with masses of 570, >210, and 300 M. and average volume densities of 1.4, > 1, and 2 x 10(4) cm (-3). We detect (CS)-S-34 (3-->2 emission in two of the clumps, indica ting peak densities in excess of 5 x 10(5) cm(-3) an attained in the c lumps. A comparison of the (CO)-O-18 line data with the 20 cm continuu m image suggests that the molecular clumps are being photoevaporated t hrough their direct exposure to the UV radiation from neighboring OB s tars, The luminosity and extent of the observed 20 cm emission is in g ood agreement with models of photoevaporative flows. We use these mode ls to estimate the pressure exerted on the clumps by the ionized gas a nd find that it exceeds the internal, turbulent pressure of the clumps by a factor of a 2.5. Although a pressure equilibrium is not excluded given the uncertainties inherent in determining the pressures of the ionized and molecular gases, our best estimates of the clump and flow parameters favor the existence of low velocity shocks (1.5 km s(-1)) i n the clumps. The clumps exhibit broad, non-Gaussian line shapes and c omplex kinematical structures suggestive of shocks. Further evidence f or shocks is found in a comparison of position-velocity diagrams with published numerical simulations of imploding spherical clumps. We disc uss the possibility that the knots of (CS)-S-34 (3-->2) emission trace gas compressed by converging shock waves. The K'-band observations sh ow a rich cluster of primarily low mass stars in the H II/molecular in terface, which we argue is divided into two distinct sub-clusters. We associate one sub-cluster with the two clumps nearest the OB stars, an d the second sub-cluster with the third clump. The two clumps nearest the OB stars contain an embedded stellar population, suggesting that s tar formation is ongoing. We discuss the impact photoevaporation is ha ving on star formation in these two clumps. We find that photoevaporat ion is dispersing the molecular gas from which the cluster is forming and estimate that the molecular gas will be completely evaporated in 2 .5 Myr. Deep K'-band imaging of the two clumps show that the stars are detected primarily on the sides of the clumps facing the OB stars and in the adjoining H II region. We examine three explanations for this asymmetry: the acceleration of the molecular clumps away from the star s by photoevaporation (i.e., the rocket effect), the unveiling of youn g embedded stars by ionization-shock fronts, and the triggered formati on of stars by shocks advancing into the clumps. if shocks do indeed e xist in the clumps, then we argue that shock triggered star formation is the best explanation of the asymmetry. (C) 1997 American Astronomic al Society.